As the game progresses you'll earn money and, as with the traditional tower defense model, there will be different types of defenses to push past. The game does stay loyal to the theme by making it so you don't have to wipe out every unit along the way and instead one just has to achieve an objective which may consist of simply reaching a destination point, knocking out a few specific units, or ... etc. With the money you earn the game does, in later stages, let you build your own army of units although it's not for awhile that you really gain any real control of that (i.e. if I have $350 and there are two units to choose from... one costs $100 and another $125 then I'm probably going to take 2 $125s thinking they must be a bit better and a $100... not too much choice). The game also adds, and this is probably the biggest thing, abilities to your arsenal as you play such as smoke screens which is the first extra ability you'll receive. I won't give away more than that as picking up new abilities is a good chunk of the adventure in playing this game. Finally, I should mention, that you can alter your path through the city at any time during the game play should you need to make an additional adjustment after your units are initially dropped onto the map.
On audio/visual alone this game is great. Units are nice and detailed. The map provides a beautiful 3D overview that you can scroll around on. The game also features different skill levels. I've focused on the easiest mode which is called Casual simply to make maximum progress in the game before penning a review.